A current microwave landing system (MLS) includes a phased array antenna for developing an electromagnetic beam which sweeps to and fro symmetrically about an airport runway centerline for guiding an aircraft in azimuth (AZ). In addition, a sector radiation pattern provides a data signal which identifies the transmission and synchronizes an aircraft's MLS receiver to the sweeping pattern of the swept beam. Furthermore, separate out-of-coverage indication (OCI) antennas radiate sector beams angled symmetrically about the runway in regions of space beyond the range of the swept beam. The OCI beams provide reference signals in one or more preassigned time slots to an airborn MLS receiver. The OCI beams are of greater amplitude in out-of-coverage sectors than unwanted direct sidelobe radiation or reflections of the swept beam from irregularities of terrain, such as nearby buildings. The OCI reference signals are employed by the MLS receiver to determine when the aircraft is within an out-of-coverage region to allow the MLS receiver to disregard such unwanted reflections of the swept guidance beam or sidelobe radiation.
These antennas are usually arranged in a cluster beyond the far end of a runway, so as to transmit the swept beam down the runway towards an incoming aircraft. Included within the landing system is timing circuitry which synchronizes the radiation of signals from the various antennas. A sequence of time slots is allowed for the generation of OCI signals from the OCI antennas. The OCI time slots are followed by one cycle of a sweeping of the swept beam which is employed by the MLS receiver to attain azimuth angle of the aircraft relative to the runway centerline. The foregoing sequence of transmissions is continually repeated so that the MLS receiver can continually update the aircraft's position.
The foregoing description applies to the azimuthal guidance of an MLS, it being understood that further signals and antennas are employed for the elevation guidance function. However, the invention pertains only to the azimuthal guidance function. Accordingly, the additional signals and antennas employed for elevation guidance function are omitted in this description of the MLS to facilitate the description of the invention.
In such prior art MLS, since separate antennas are used for AZ guidance and OCI it increases the cost and complexity of the MLS.
It is, therefore, an object of the present invention to provide a new and improved MLS ground system which is simpler, has lower cost and is more versatile than prior MLS ground systems, and has increased OCI capability.